The main reason why they don't allow you to tether your phone is not because they just want you to pay twice, although I'm sure that has some play into it as well, but most people won't pay for it anyway, but because they don't want people to actually use the data amount they were given.
Right now part of their business model is that most people do not use the whole 2 or 3 GB of data that they are given. Allowing them to tether, even though it would bring them more money if the user goes over the limit, means a lot of people will start using all the data they received.
That's a tactic many businesses use. Oversubscribe services and profit from the limited use of the product. Punish the few who actually try to make full use of the service.
See any cheap, shared hosting service. They'll promise unlimited so long as you don't try to take them up on that offer.
Certainly not a new thing. Circa 1994 my dial-up ISP (with "unlimited" internet, the first one in the area) was oversubscribed like every dial-up ISP, they had an order of magnitude less connections available than paying customers. When I actually set the connection to stay up 24/7, they shut the account down within a week.
Since you speak of bandwidth by monthly use instead of transfer rate per second, i take it that you understand as much of the subject as phone operators.
If you sell by anything other than slicing your pipe width, you're selling more than you have. Period.
I think you'll be hard pressed to find any ISP that doesn't oversell (at least to consumers - business connections are a different issue). In fact, it'd be extremely inefficient to have all that reserved bandwidth when only an extremely small slice is being used at any time, not to mention that the prices would probably go up ten fold or more.
Overselling is not a problem if the ISP manages it well (keeping a decent window to account for expect and some unexpected growth) and don't punish their users if they use what was promised.
Right now part of their business model is that most people do not use the whole 2 or 3 GB of data that they are given. Allowing them to tether, even though it would bring them more money if the user goes over the limit, means a lot of people will start using all the data they received.